U.S. patent number 7,413,194 [Application Number 11/258,183] was granted by the patent office on 2008-08-19 for pressure balanced annular seal.
This patent grant is currently assigned to Rolls-Royce PLC. Invention is credited to Gervas Franceschini, Christopher Wright.
United States Patent |
7,413,194 |
Wright , et al. |
August 19, 2008 |
Pressure balanced annular seal
Abstract
This invention provides a segmented annular seal (10) the
sealing leakage gaps between relatively movable components 14,16
between high and low pressure regions in a gas turbine engine, for
example for sealing an engine shaft 16. The seal comprises an
annular array of circumferentially spaced and radially movable
arcuate seal segments (12) which extend in a continuous end to end
relationship in annular housing (14) and project radially from the
housing for effecting a seal against an engine shaft (16), rotor or
the like. The segments (12) are located in a channel (22) in the
housing (14) sized such that the segments (12) are able to follow
radial movements of the shaft (16). In order to reduce an axial
loading on the segments (12) that ends to cause them to seize a
pressure balance region (30) vented to the upstream pressure region
is formed on the downstream side of the segments (12). A second
pressure balance region (42) vented to the downstream pressure
region may be formed on the upstream side of the segments (12) to
relieve a twisting moment acting on the segments.
Inventors: |
Wright; Christopher (Bristol,
GB), Franceschini; Gervas (Exeter, GB) |
Assignee: |
Rolls-Royce PLC (London,
GB)
|
Family
ID: |
56409409 |
Appl.
No.: |
11/258,183 |
Filed: |
October 26, 2005 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060208427 A1 |
Sep 21, 2006 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 28, 2004 [GB] |
|
|
0423923.2 |
|
Current U.S.
Class: |
277/355; 277/422;
277/431; 277/579; 277/580; 277/581 |
Current CPC
Class: |
F16J
15/3288 (20130101); F16J 15/3292 (20130101); F16J
15/441 (20130101) |
Current International
Class: |
F16J
15/44 (20060101); F16J 15/00 (20060101) |
Field of
Search: |
;277/355,543-545,548,581,422,431,579,580 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2393766 |
|
Apr 2004 |
|
GB |
|
A 11-257013 |
|
Sep 1999 |
|
JP |
|
Primary Examiner: Engle; Patricia
Assistant Examiner: Lee; Gilbert Y
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
The invention claimed is:
1. A segmented annular seal assembly comprising an annular seal
ring consisting of a plurality of arcuate seal segments disposed
end-to-end to form an annular seal operative in use to seal a
leakage gap between a fixed part and a relatively rotatable movable
part, wherein, in use, there is an upstream pressure region of high
pressure on one side of the annular seal and the fixed part and a
downstream pressure region of lower pressure on the other side of
the annular seal and the fixed part, the fixed part having an
upstream wall and a downstream wall that define a recess within
which the seal segments are disposed, the upstream wall and the
downstream wall being spaced apart such that the seal segments are
free to follow radial movement of the movable part relative to the
fixed part, at least a portion of each of the seal segments being
in sliding contact with at least a portion of each of an inner face
of the upstream wall and an inner face of the downstream wall of
the fixed part, the inner face of the downstream wall of the recess
being formed with a first pressure balance region within the recess
adjacent the downstream side of the seal segments, the inner face
of the upstream wall of the recess being formed with a second
pressure balance region within the recess adjacent the upstream
side of the seal segments, and a first communication passage
through the fixed part linking the first pressure balance region in
the downstream wall of the recess with the upstream pressure region
via an opening in the outer face of the upstream wall, and a second
communication passage, separate from the first communication
passage, through the fixed part linking the second pressure balance
region in the upstream wall of the recess with the downstream
pressure region via an opening in the outer face of the downstream
wall.
2. A segmented annular seal assembly as claimed in claim 1 further
comprising first secondary seal means between the downstream wall
of the recess and the seal ring whereby to isolate the first
pressure balance region from the downstream pressure region.
3. A segmented annular seal assembly as claimed in claim 2 wherein
the first secondary seal means is selected from a group of seal
means including lip seals, brush seals, leaf seals, and elastomeric
seals.
4. A segmented annular seal assembly as claimed in claim 1 further
comprising second secondary seal means between the upstream wall of
the recess and the seal ring whereby to isolate the second pressure
balance region from the upstream pressure region.
5. A segmented annular seal assembly as claimed in claim 4 wherein
the second secondary seal means is selected from a group of seal
means including lip seals, brush seals, leaf seals, and elastomeric
seals.
6. A segmented annular seal assembly as claimed in claim 4 wherein
the first and second pressure balance regions are isolated from
each other by secondary seal means within the recess.
7. A segmented annular seal assembly as claimed in claim 1 wherein
the first pressure balance region comprises at least one recess or
pocket formed in the downstream wall of the recess.
8. A segmented annular seal assembly as claimed in claim 1 wherein
the first second pressure balance region comprises at least one
recess or pocket formed in the upstream wall of the recess.
9. A segmented annular seal assembly as claimed in claim 1 wherein
the arcuate seal segments are biased towards the relatively
rotatable part.
10. A segmented annular seal assembly as claimed in claim 1 wherein
the annular seal ring is selected from a group of seal means
including abradable contact seals, brush seals, leaf seals, and
air-riding seals.
Description
BACKGROUND
This invention relates to a pressure balanced annular seal. In
particular the invention concerns a segmented annular seal assembly
for sealing between relatively movable components in a gas turbine
engine.
The invention has for a principal objective the reduction or
elimination of pressure loading on seal segments in the direction
of main gas flow by use of pressure balance features acting in the
opposite direction to the main direction of gas flow.
The relatively movable components of the present invention are
commonly relatively rotatable, but in this case movable is intended
to embrace lateral or transverse movement as well as rotational
movement. It will be understood that, in use, a turning shaft,
rotor or the like may also be subject to a certain amount of
lateral or transverse movement. Therefore it will be understood
also that such a seal will be effective even when the shaft rotor
or the like is rotationally stationary.
A segmented annular seal assembly is known from U.S. Pat. No.
6,669,203 in which a plurality of arcuate brush seal segments are
disposed in a radially inwardly opening groove of a stationary
component to engage the surface of a relatively rotating component.
Each brush seal segment has limited radial and axial clearances
with respect to the fixed housing provided by a hook or flange
carried on the upstream side of each segment engaged with a slot
formed in the housing. A spring is disposed between the segment and
the base of the groove and biases the seal segments towards the
rotating component so as to follow radial movements. It is claimed
that this arrangement permits at start up high pressure flow on the
upstream side to bias the segments in an axial downstream direction
and also permits the high pressure to enter into the base of the
groove and the radially outer face of the segments to bias the
segment radially inwardly to ensure proper sealing of the bristle
tips along the rotor surface.
However, a disadvantage of the arrangement is that friction between
the downstream faces of the segments and the downstream wall of the
groove due to the high pressure on the upstream side can cause the
segments to stick to the wall and fail to track radial excursions
of the rotor leading to increased seal leakage. This can lead to
uneven and excessive seal wear, and it is impracticable to employ a
bias force sufficient to overcome the sticking force as it would
increase seal wear to unacceptable levels.
In U.S. Pat. No. 6,572,115 a segmented seal is mounted in a seal
carrier which is movable in radial directions to accommodate radial
transients. The seal carrier segments are disposed in a channel
creating a space on the radially outer side of the segments to
which upstream pressure is supplied thereby create a bias force
acting radially downwards to bias the seal portions into contact
with the rotor. However, the structure of the embodiments disclosed
exposes the upstream facing faces of the movable seal parts to
upstream pressure, and the downstream facing faces of the movable
seal parts to downstream pressure. Consequently the seal carrier
segments are subject to a tilting force as a result of the
difference in pressure between the upstream and downstream sides.
This may lead to the segments jamming in the channel so that they
cannot follow radial transient movements and suffer uneven and
premature wear as a result.
In such prior art arrangements as these shaft seal segments which
slide (radially in the case of axial flow, axial in the case of
radial flow) suffer from high contact loading associated with the
net pressure differential across them. The imbalance in pressure
across the seal causes a net force in the direction of fluid flow.
The segments then seize against the retaining plate/ring and
segment movement is either very difficult or prohibited. The
additional forces required to move the segments manifest themselves
as higher loading between the seal and a relatively rotatable part
or a static structure, in the case of a static seal. Where there is
relative movement high wear results.
SUMMARY
The present invention is concerned with improving the performance
of a seal between regions of different pressure, for example: to
prevent oil leakage from the engine bearing chambers in gas turbine
engine. Such seals require a degree of radial compliance to
accommodate transient radial excursions, for example, due to shaft
and/or rotor eccentricities and/or radial growth due to centrifugal
forces and/or differential temperatures. The invention concerns
pressure-balancing means to offset the pressure loading on
components of the seal in the direction of fluid flow. Lower forces
result and the segments are then able to move with relative ease,
much improving the overall life of the seal.
The primary sealing mechanism, that is the type of components that
provide the seal between relatively movable parts, is not crucial
to the present invention. So the primary seal, for example, may
comprise a brush seal, an abradable face seal, an air riding seal,
leaf seals and the like or any other type of suitable seal.
According to an aspect of the present invention, there is provided
a segmented annular seal assembly for sealing a leakage gap between
regions of different pressure comprising a fixed part and a
relatively rotatable movable part, the fixed part and the movable
part being formed to define between them the leakage gap between an
upstream region of high pressure and a downstream region of lower
pressure, an annular seal ring disposed to seal the leakage gap,
the seal ring consisting of a plurality of arcuate seal segments
carried by the fixed part within a recess formed with an upstream
wall and a downstream wall within which the seal segments are
carried, the upstream wall and the downstream wall being spaced
apart such that the ring segments are free to follow movement of
the movable part relative to the fixed part, characterised in that
there is provided a first pressure balance region formed adjacent
the downstream wall and the seal ring and communication means
linking the pressure balance region with the upstream pressure
region on the upstream side of the fixed part whereby to tend to
equalise pressure in said regions.
The annular seal of the present invention readily enables a
radially compliant seal to be provided in a gas turbine engine for
effecting a seal against a rotatable or stationary engine shaft or
engine rotor assembly. The radially movable primary seal segments
are slidably located in the housing so that they are capable of
accommodating significant radial interference due to shaft and/or
rotor eccentricities and/or radial growth due to centrifugal loads
and/or differential thermal expansion. The biasing force provided
by the biasing means is preferably great enough to maintain the
primary seal segments in contact with the shaft or other component
being sealed with minimum contact force so that the segments are
radially movable with minimum force to minimise wear of the
components due to the contact load. Circumferential movement of the
primary seal segments is restrained by the guide means to prevent
circumferential movement of the segments in the housing.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and how it may be carried into practice will now be
more particularly described, by way of example only, with reference
to the accompanying drawings, in which:
FIG. 1 is an end view of an annular segmented seal of known type
viewed in the axial direction of the seal;
FIG. 2a is a cross-section view of the seal of FIG. 1 along the
line I-I of FIG. 1;
FIG. 2b is a diagram illustrating the pressure forces acting on the
seal components in use;
FIG. 3 shows in more detail a plurality of seal segments of the
seal of FIGS. 1 and 2, viewed in the direction II in FIG. 2;
FIG. 4 shows the overlapping arrangement of the circumferential
ends of adjacent segments viewed in the direction of III in FIG.
3;
FIG. 5a is a cross-section view similar to FIG. 2a of a seal in
accordance with the present invention having a downstream pressure
balance region;
FIG. 5b is a diagram illustrating the pressure forces acting in use
on the seal components of FIG. 5a;
FIG. 6a is a cross-section view similar to FIG. 2a of a seal in
accordance with the present invention having downstream and
upstream pressure balance regions;
FIG. 6b is a diagram illustrating the pressure forces acting in use
on the seal components of FIG. 6a;
FIG. 7a is a cross-section view similar to FIG. 6a of a seal in
which the downstream and upstream pressure balance regions are
defined by a first form of secondary seals within the seal
receiving channel;
FIG. 7b is a diagram illustrating the pressure forces acting in use
on the seal components of FIG. 7a;
FIG. 8a is a cross-section view similar to FIGS. 6a and 7a in which
the downstream and upstream pressure balance regions are defined by
an alternative form of secondary seals within the seal receiving
channel;
FIG. 8b is a diagram illustrating the pressure forces acting in use
on the seal components of FIG. 8a; and
FIG. 9 is a cross-section view similar to FIGS. 7a and 8a of a seal
in accordance with the present invention having an alternative form
of primary seal.
EMBODIMENTS
In the following description the terms: primary seal is used in
reference to the seal against the rotating shaft (i.e., the
original sealing interface); and secondary seal is used in
reference to the low duty seals located between the static seal
(segments) and the static seal housing.
Referring now to the drawings, in which like parts carry like
references, in FIG. 1 there is shown part of the circumference of
an annular segmented primary seal ring 10 for sealing between
relatively rotatable components for example in a gas turbine engine
or the like. Annular seals of the kind referred to above may be
used for sealing leakage gaps between relatively rotatable
components in any type of rotating machinery. Although the
invention is particularly useful for sealing between relatively
rotatable components, it will also be appreciated that is not its
exclusive use. The seal is intended to accommodate a certain amount
of transverse or lateral relative movement of the components
between which a seal is being effected. Thus, it follows that the
seal will remain operative when the components are relatively
rotationally stationary so it may be used as a seal between
components that are relatively non-rotational but remain capable of
relative transverse or lateral movement. Consequently, references
to shafts and rotors in the following description are to be
construed accordingly.
The full circumference of the annular seal ring 10 is identical to
the part of the circumference shown in FIG. 1. The seal ring 10
comprises an annular array of circumferentially spaced and radially
movable arcuate seal segments 12 which extend in a continuous end
to end relationship in an annular housing 14. The seal segments
project radially inwards from the housing 14 for effecting a seal
against an engine shaft, as indicated by lines 16, about which the
seal is co-axially and concentrically disposed.
Referring now to FIG. 2a, the housing 14 has a generally U-shaped
cross-section in the circumferential direction of the seal formed
by opposing annular members 18, 20 including a first member or
front ring 18 on the upstream, high pressure side and a second
member or backing ring 20 on the downstream or lower pressure side.
Between them the members 18, 20 define a continuous circumferential
channel 22 in the interior of the housing 14 in which the arcuate
segments 12 are slidably located. The channel 22 extends radially
from the radially outer part of the housing that defines the base
of the U to the opening 24 at the radially inner part of the
housing. The lower face 25 of the housing 14 is spaced from the
surface of the relatively movable component 16 by a distance "c"
which thereby defines the leakage gap to be sealed. The axial width
dimension of the channel 22 between the respective upstream and
downstream axial side faces 26 and 28 of the members 18 and 20 is
substantially the same as the width dimension of the segments 12 so
that the segments have a sliding fit in the channel 22 in which
they are located. Axial movement of the segments 12 within the
housing is therefore substantially prevented. The seal segments 12
have a radial depth within channel 22 of "1" and the corresponding
radial depth of the U-shaped channel 22 is greater than "1" by at
least the maximum radial excursion to be accommodated.
Each of the segments 12 is guided in the channel 22 by an
inter-engaging pin and slot arrangement, comprising a radial slot
30 in the segments and an axially extending guide pin 32 carried by
the end member plate 20. The slot 30 and pin 32 act to guide the
segments in the radial direction of the seal and restrain the
segments against circumferential movement within the housing. The
segments 12 preferably are radially biased with respect to the
housing in this illustration by bias means represented by a spring
element 34. The spring element may comprise, for example a
circumferential garter spring, positioned between the base of the
channel at the closed end thereof and the radially outer end of the
respective segment. The springs 34 bias the segments 12 radially
inwards towards the shaft 16 or other component against which they
seal.
FIG. 2b illustrates the pressure loading experienced by the seal
segments 12 due to the pressure differential .DELTA.p between the
pressure p.sub.up in the upstream region to the left of the seal
segments 12 and the pressure p.sub.down in the downstream region to
the right of the seal segments. The magnitude and direction of the
force exerted by the pressure differential .DELTA.p is indicated by
the size and direction of the arrow in FIG. 2b. As is represented
in the drawing, the force is substantial constant across the width
of the leakage gap "c" and reduces to zero within the channel 22
with increasing distance inside the housing. The force is uniform
around the circumference of component 16 and is directed axially
through the leakage gap from the higher pressure, upstream region
towards the lower pressure, downstream region. Possible tangential
components due to swirl in the vicinity of the leakage gap arising
as a result of relative rotation have been ignored for the purposes
of this description.
The slot and pin arrangement is illustrated in the view of FIG. 3
which shows a plurality of adjacent arcuate seal segments 12. In
the view of FIG. 3, the surrounding detail of the housing is
partially cutaway so that the inter-engagement of the pins 32 with
the slots 30 can be seen. As shown in FIGS. 3 and 4, the respective
circumferential ends of the segments are provided with overlapping
end portions 36 so that the circumferential ends of adjacent
segments in the seal overlap to form sliding lap type joints. This
arrangement enhances the sealing effectiveness of the segmented
annular seal, reducing inter-segment leakage flow to a very low,
negligible level. The pin and slot guide arrangement, and similarly
the bias spring means, are shown as exemplary only. A number of
alternatives for each will be apparent to a reader skilled in the
art and may be utilised in the described arrangement within the
scope of the presently claimed invention.
Improvements to seals of this kind in accordance with the present
invention are illustrated the drawings of FIGS. 5 to 9. The basic
principal of the invention is illustrated in FIG. 5, and the
remaining figures of the drawings show enhancements or variations
of features of the invention.
Referring to FIGS. 5a and 5b, the invention has for a major
objective the reduction or elimination of pressure loading on seal
segments 12 in the direction of main gas flow by the use of
pressure balance features acting in the opposite direction to the
direction of main gas flow. This is achieved by the provision of a
pressure balance cavity, recess or pocket, generally indicated at
38, in the downstream member 20 which thereby provides an upstream
directed force to offset some of the pressure loading on the
segments 12.
The seal arrangement of FIG. 5a is modified relative to that shown
in FIG. 2a by the provision of at least one pressure balance
cavity, recess or pocket 38 within the limits of the U-shaped
channel 22 on the downstream side of the seal segments 12. Upstream
pressure p.sub.up is conducted to the cavity 38 through the
interior of the housing 14 to the low pressure side of the segments
12.
In the particular embodiment shown in FIG. 5a, the cavity, recess
or pocket 38 is provided in the upstream side face 28 of the
downstream housing member 20 by increased width (i.e., the
dimension in the axial direction) form the base of the channel 22
over most of its radial depth. Only a lip 48 remains at the
radially inner circumference of the member 20 to provide a seal
against the downstream side of the seal segments 12. The lip 48
effectively forms a secondary seal to prevent, or minimise leakage
from the cavity 22, 38. In other embodiments, the cavity 38
comprises a plurality of individual recesses spaced apart
circumferentially around the annular channel side face 28. The
upstream pressure p.sub.up is conducted to the cavity 22, 38
through a plurality of circumferentially spaced passages 40 formed
through the upstream side member 18 in the region of the base of
the channel 22. The passages 40 convey high pressure air from the
high pressure region on the upstream side of the housing 14 to the
channel 22 and to the respective recess or recesses 38.
Irrespective of the number of such recesses provided the seal
segments 12 are pressure loaded towards the downstream side face 26
of the upstream member 18 during operation. The axial force
generated on the seal segments 12 within the channel 22, as shown
in FIG. 5b, is thereby effectively balanced, to a greater or lesser
extent, to generate a net zero pressure induced force acting on the
seal segments 12 over the radial depth of the channel 22. However,
there remains an axial force and a moment exerted on the seal
segments 12 as a result of the axial force due in the leakage gap
"c" due to the pressure differential .DELTA.p.
Considering the segments as sufficiently small for linear
approximation: Net pressure side load (N) and moment (M) is,
generally given by the equations:
.intg..times..times..times..DELTA..times..times..times.d.times..times..in-
tg..times..times..times..DELTA..times..times..times.d
##EQU00001##
The net pressure loading (N) without any features, i.e., where the
seal segments 12 fit closely within their receiving channel 22 as
in the prior art is difficult to determine because the pressures
between the faces are not well defined. However, to a first
approximation, if the pressure everywhere else is the same, then
N=.DELTA.pc.
In reality the pressure on the whole upstream side is likely to be
close to upstream pressure p.sub.up, and that on the whole
downstream side is likely to be close to the downstream value
p.sub.down.
Where there is a linear pressure drop along the unexposed part of
the seal segments 12, then
.DELTA..times..times. ##EQU00002## where "l" and a "c" are the
dimensions indicated in the drawings.
Essentially, prior art segmented seals have a loading which, for a
typical segment of order 10 mm length, 1 mm clearance and 1 MPa
pressure drop, is of the order of 5500 N/m. If friction is of the
order of 0.1, then force required to move a "radially movable
segment" is around 550 N/m. The drawback with the prior art,
therefore, is that segmented seals cannot be operated effectively
at moderate pressures without incurring significant wear against
the rotating shaft. In these prior arrangements the moment on the
seal segments
.DELTA..times..times. ##EQU00003##
In arrangements embodying the invention with high pressure fluid
fed to the balance region as in FIG. 5a, the force reduces to
N=.DELTA.pc
In this case, the net pressure load is of order of 1000 N/m, giving
the force required to move a segment as 100 N/m, that is a force
five times lower than in an equivalent arrangement according to the
prior art. The moment is also reduced to
.DELTA..times..times. ##EQU00004##
Comparing the two moments there is a reduction in the ratio prior
art/invention in the proportion of 9 to 4.5.
In the embodiment of FIG. 6a, in order to offset the load even
further, there is provided at least further cavity, recess or
pocket 42 on the upstream side of the seal segments 12 vented to
the downstream, low pressure region through further pressure
communication means 46. The cavity, recess or pocket 42 may be
formed as a single annular cavity or recess, or as a plurality of
recesses or pockets in the side face 26 of the channel 22, that is
in or against the downstream side face 26 of the upstream housing
member 18. The radially inner edge of the cavity, recess or pocket
42 is defined by a lip 49 formed on the radially inner edge of the
housing member 18. This lip 49 may be arranged at the same radial
height as the lip 48 of the opposing housing member 20, but this is
not essential and generally is not so arranged.
The pressure communication means 46 comprises a plurality of holes
formed in the housing 14. These holes 46 open into the low pressure
region on the downstream side of the housing, and in passing
through the housing 14 do not intersect the corresponding high
pressure communication passages 44 nor the channel 22. The net load
can then be reduced to, or close to, zero, by appropriate sizing of
the balance pockets 38, 42. The size of the pressure communication
passages 46 is relatively unimportant, providing leakage flow rates
are sufficiently low or negligible.
FIG. 6b illustrates the forces acting on the seal segments 12 in
this arrangement. As before the size and direction of the arrows
representing the forces due to the several pressure regions on
different parts of the seal segments. The greatest force due to the
pressure differential .DELTA.p is exerted in the downstream
direction over the area of the seal segments exposed in the leakage
gap. The force exerted over the area on one side of the seal
segments exposed to pressure p.sub.bal in the downstream balance
pockets 38 is nearly (but not quite) p.sub.up and acts in the
upstream direction. The force exerted over the area on the opposite
side of the seal segments exposed to pressure p.sub.vent in the
upstream balance pockets 42 is nearly (but not quite) p.sub.down
and acts in the downstream direction. Although the forces in the
pockets 38, 42 act in opposite directions and can be arranged to
nearly cancel each other, a net moment due to the radial separation
of leakage gap force will remain tending to twist the segments.
However, in general there exists a sealing system comprising a
number of carefully chosen balance pockets, and including a
staggered relation of pressure communication passages 44,46 spaced
apart circumferentially around the seal segments 12 that can,
theoretically yield N=0 and M=0 at the same time.
With reference to FIG. 7a, the internal dimensions of the channel
22 are enlarged overall to the depth (i.e., dimension in the axial
direction) of the pressure balance pockets. The upstream and
downstream pressure balance cavities 42, 38 are created by
secondary, circumferentially extending seal means 50 on the
upstream side of the seal segments 12, and 52 on the downstream
side respectively, and by the radially inner lips 49, 48. The
corresponding pressure supply passages 44, 46 are sized
appropriately to take account of the expected leakages through the
secondary seals, hence p.sub.bal is nearly (but not quite)
p.sub.up, p.sub.vent is nearly p.sub.down. An additional space 54
is created in the base of the channel 22 bounded by the secondary
seals 50,52. The pressure p.sub.spr in this space is close to the
mean pressure under the primary seal due to leakage under the
secondary seals. Thus, where subscripts "F" and "R" refer to front
and rear (in the sense of upstream and downstream)
respectively:
##EQU00005## ##EQU00005.2## ##EQU00005.3##
Therefore, by judicious selection of p.sub.vent, p.sub.bal,
c.sub.F, c.sub.R, d.sub.F and d.sub.R the force N and moment M on
the segments can be minimized over the range of movements expected.
The determination of the vent pressure p.sub.vent in the upstream
cavity 42, and balance pressure p.sub.bal in the downstream cavity
38 comes from a mass flow balance of the whole sealing system
taking account of the flow through the communication passages 44,
46 and leakage between the segments. FIG. 7b illustrates by the use
of arrows indicating the magnitude and direction of the various
pressure forces acting over the whole surface of the seal
segments.
FIG. 8a shows another embodiment of the invention, similar to that
of FIG. 7a in which the secondary seals 50, 52 and 48, 49 comprise
large diameter, annular brush seals in which the seal bristles are
laid in a cylindrical surface at the seal radius. Otherwise the
brush seals may be of conventional construction. FIG. 8b
illustrates the forces acting on the seal segments 12.
Theoretically this force diagram is identical with the force
diagram of FIG. 7b.
As previously mentioned the primary seal formed or carried by the
seal segments 12 may comprise one of a number of different types of
suitable seal, the range of suitable seals including abradable
contact seals, brush seals, leaf seals, air-riding seals or the
like. In the previous Figures illustrating the invention no
specific form of primary seal has been indicated. In FIG. 9a the
hatched area 60 indicates how a seal such as a brush seal may be
mounted in the seal segments 12 for sealing against a shaft 16. As
in the embodiment illustrated in FIG. 8a the secondary seals 50, 52
also comprise brush seals as previously described. The
corresponding pressure force diagram is shown in FIG. 9b.
Although aspects of the invention have been described with
reference to the embodiments shown in the accompanying drawings, it
is to be understood that the invention is not limited to those
precise embodiments and that various changes and modifications may
be affected without further inventive skill and effort. For
example, where particular features have been shown and described in
one embodiment it is understood that various combinations of those
features with all or some features of other embodiments could be
readily achieved without exercising further skill and effort and as
such are contemplated by the present invention.
* * * * *